There is no such human, as a former Christian.There can be a nominal person, who calls their self a Christian, then for whatever reason then say they no longer believe in God.
Why do I say this? The Scripture is perfectly clear that if a person is born again [John 3:3,5-1 Peter 1:3,22-25] they are sealed into Salvation [John 6:39-40; 2 Corinthians 1:21-22-Ephesians 1:13-14].
So if one is an Atheist after calling their self a Christian, they were a Christian in name only.

Assuming that ‘go to heaven’ means being saved, I will say that only God will make that determination in the judgment day (Matt 13:49; Heb 9:27; 2 Cor 5:10).
Moreover, ‘former Christian’ refer to a person who accepted Christ as personal Savior, made public confession of faith, got baptized and later on changed his/her mind about following Christ. In the parable of the sower, Jesus describes exactly the people who at some point believed the Gospel, welcomed Christ in their lives but later on they ‘withered away’ - something that was once living had died. This clearly means that some people have received salvation at some point but lost it because they did not grow deeper roots in Christ. (Matt 13:3-7).
Christians do not lose their free-will. God does not take hostages. King Saul was Spirit-filled but died a lost man. Judas Iscariot was one of the 12 disciples who Christ sent out to preach the Gospel (along with 70 others), and at some point was filled with the Holy Spirit (Luke 10:17). Judas was among the 12 and Christ wanted his name engraved in the foundation of the New Jerusalem (Rev 21:14). However, Judas allowed Satan to use him and lost his salvation (Luke 22:3).
People are saved by grace, through faith in Jesus Christ (Eph 2:8). The Bible clearly teaches that all who believe in Christ will be saved (Acts 16:31; Rom 10:9).
However, by definition, an atheist does not believe in God’s existence. Knowing that he only way to be saved is by faith in Christ (Acts 4:12), can an atheist be saved?
‘Whoever has the Son has life; whoever does not have the Son of God does not have life’ (1 John 5:12).

I have a take for this question that correlates with many others and is also an important Central point of emphasis of understanding scripture. Professing one's faith and belief in Jesus Christ and asking God's forgiveness of all of their many sins is the very moment that your many sins will be removed "as far as the east is from the West"! ***The answer is a profound YES!*** From that event, one is permanently cleansed of their unrighteousness. Satan and the world will overload you with temptations, trials and tribulations at every turn and every single opportunity! Jesus told us each that "we will suffer for His namesake" and that we are "to stand strong in the face of adversity by not succumbing to the wiles of Satan" People that are living as the world wants are the one's seemingly cruising through life in a perfect state of tranquility with no problems, cares or concerns. These souls are in a state of serious eternal life residence situation! The answer lies in this truth from God's Holy Scripture: "For as long as ye have breathe on this Earth, ye can call out to God, the Father, in the name of Jesus Christ, His One and Only Begotten Son to request to be forgiven of all your many, many sins and "Ye shall be forgiven and have all your sins washed away "as far as the east is from the west". (There have been many 'last breathe saving moments of surviving spouses/family/friends witnessing their loved one verbally expressing their request at that 'final hour moment of repentance to God in Heaven'.)
"For God so loved the world that He gave His only begotten son, that whosoever believes in Him shall not perish, but have life everlasting/eternal".
"Praise God that He provided us a savior. And, His name is Jesus Christ! --ANDY--

Based on scripture, yes. People can get confused. I would think it would be rare, but possible at the same time.
What if as a child they believed the simple gospel of John 3:16-18. At that point, they were sealed by the Holy Spirit of promise. Ephesians 1:13-14. Once we have that seal, we have to go to heaven.
But as they grew older they were being told that what they believed was foolish, and they began to believe it.
Second Timothy 2:13 If we believe not He abideth faithful, He can not deny himself. His Holy Spirit that lives in all believers. God may chasten that person and try to bring them back to the truth, and He may take them out of the world as useless to Him. But, they are still saved. They may lose their life, and their rewards in heaven may be burned up, but they are still saved so as by fire.
First Corinthians 3:12:21

The Bible is very clear:
HEBREWS 6:4-8
4 For it is impossible for those who were once enlightened, and have tasted the heavenly gift, and have become partakers of the Holy Spirit,
5 and have tasted the good word of God and the powers of the age to come, 6 if they fall away, to renew them again to repentance, since they crucify again for themselves the Son of God, and put Him to an open shame.
7 For the earth which drinks in the rain that often comes upon it, and bears herbs useful for those by whom it is cultivated, receives blessing from God;
8 but if it bears thorns and briers, it is rejected and near to being cursed, whose end is to be burned.

I think it'll help to analyze this question step by step. Can one who is born again by believing the gospel, which asserts that Jesus, born of a virgin by the Spirit of God and not by the seed of a man, was sent by God to die on a cross for the sins of mankind, can he become an unbeliever? If so, then "born again" unto eternal life is a misnomer. Eternal life can't be halted and still be eternal.
Atheism is a belief that no deity exists to believe in. Is it possible to believe in Jesus and be born again, and then be unborn, deciding there's no one to believe in? If your sins have been forgiven can they be reinstated and put back to your sin account?
Take care brothers, lest there be in any of you an evil, unbelieving heart, leading you to fall away from the living God (Heb 3:12). Vs 13-18 explains vs 12. Vs 19 blurts it out: "So we see that they (those who heard the voice of God and rebelled because of the deceitfulness of sin) were unable to enter (God's eternal rest) because of unbelief (Heb 3:19).
For it is impossible, in the case of those who have been enlightened, to have tasted the heavenly gift, and have shared in the Holy Spirit, and have tasted the goodness of the word of God and the powers of the age to come, and then have fallen away, to restore them again to repentance, since they are crucifying once again the Son of God to their own harm and holding Him up to contempt (Heb 6:4-6). This doesn't mean they "fell away" through unbelief. This means repentance only occurs once, so it's time for these "Hebrews" to make up their minds about their beliefs. They are stuck at the elementary stage of going from the teachings of Judaism that brought them to this phase of God's plan for His people. To not continue is to risk not receiving the blessings God intends for them. He uses an analogy of land receiving rain that produces a useful crop, compared to land that produces a worthless crop to the one who cultivated the land. The harvest is called "thorns and thistles." This has been misunderstood to mean that you can go from lost sinner, to born again believer, to lost again. This is really about advancing from Judaism to Christianity, moving on from being tutored by the Law of Moses to living in fellowship with Jesus by the help of the Holy Spirit.
Life begins with no knowledge of God whatsoever. We either remain there or we begin to believe "something," maybe Christianity. From Atheism to belief in God. There's another step, however, before you're born again. Change has occurred but is only the beginning. "Transition" is to begin. You must ask forgiveness for your sins; REPENT. To be forgiven is to be "born again" unto eternal life. Saved to never be a lost sinner again.
Submission to the Spirit of God will lead the new convert to serve God and accomplish His purpose for the believer, conformity to the image of His Son (Rom 8:29). The result is a life of service to God and others, signified by expressions of love. Rebellion is refusal to submit to being conformed, to not "finish the race," to use a Pauline metaphor.
What is this image of Christ? It is to be a humble servant willing to suffer for the purpose of God. 1 Pet 4 explains it. Many believers think 'Christlike" is "holy living." No. It's "suffering" for God's purpose.
Can a person return to a theological belief that there's no deity to believe in after believing there is? I have no idea. But if repentance didn't happen that person didn't believe unto salvation, and will not spend eternity with the Lord. Belief should lead to repentance.
"Or do you presume on the riches of His kindness and forbearance and patience, not knowing that God's kindness is meant to lead you to repentance?" Rom 2:4. Repentance is the vehicle that begins our eternal life journey. Heaven is the destination. Until that final "transition" our purpose is to be a "bumper crop" for the Lord. Thorns and thistles are only useful as fuel for fire. That's hardly an eternal purpose.

I don't think anyone who has ever truly confessed Christ as Lord and Savior would turn away from him. If that person does, then I would call into question if that person was ever truly saved at all.
Jesus said that you must confess with your mouth and BELIEVE in your heart. People can say the words and fool everyone around, but only God can read the heart, and only He knows for sure if that person truly believed the confession they just made or if they were just giving "lip service".
I think it would be impossible for anyone to experience the goodness and wonderful nature of our Lord and Savior and then turn their backs on him. But the bible does tell us the "apostasy" must take place before the end, basically a falling away of so-called believers.
Jesus said that everyone God has given to him will come to him and none would be lost. If these so-called believers turn away from Christ, them maybe they were never his to begin with.